This invention relates to a method of controlling idling rotational speed in an internal combustion engine, and more particularly to an idling rotational speed control method for coping with a sudden drop in engine rotational speed from high rpm by controlling the amount of intake air through use of a control valve provided in a bypass passage bypassing a throttle valve arranged in an intake passage of the internal combustion engine, whereby the rotational speed of the engine makes a smooth transition to a target idling speed in a feedback control mode.
When an internal combustion engine is idling or operating under a low load with the throttle valve kept in a substantially fully closed position, the conventional practice is to control the idling speed of the engine by regulating the intake air amount by means of a control valve arranged in a bypass passage bypassing the throttle valve, i.e., communicating the upstream and downstream sides of the throttle valve.
In internal combustion engines, even those equipped with an electronically controlled fuel injection system, it is commonly known that when the amount of intake air increases, there is an accompanying increase in the amount of fuel injected, which in turn results in greater supply of the mixture. According to a typical conventional method, the degree to which the control valve is opened is placed under closed-loop control when the throttle valve is substantially fully closed and, at the same time the rotational speed of the engine is in a predetermined idling speed region. More specifically, the magnitude of an excitation current supplied to a solenoid of the control valve for proportional control of the control valve opening is decided on the basis of a solenoid current command value Icmd, which is specified by the following equation: EQU Icmd=Ifb(n) (1)
where Ifb(n) represents a PID feedback control term applied for executing proportional control (P term), integral control (I term) and differential control (D term) on the basis of a difference between target idling rotational speed Nrefo and the actual rotational speed Ne of the engine.
Assume by way of example that the engine rotational speed is raised to high rpm by opening the throttle valve to a greater degree, and thereafter the throttle valve is substantially fully closed and the engine is placed in unloaded state, as by shifting the transmission to the neutral range or stepping down on the clutch pedal. This will cause the rotational speed of the engine to undergo a sudden drop. When the engine rotational speed falls to a value within the predetermined idling speed region, the opening of the control valve is subjected to feedback control in such a manner that the engine rotational speed will approach the target idling rotational speed, as mentioned above.
However, if the downward trend exhibited by the engine rotational speed is very sudden at such time that the engine is in the unloaded state, the rotational speed will temporarily drop below the target idling speed before being stabilized at this speed by feedback control.
The applicant has already proposed, in Japanese Patent Application No. 59-267508, a method of preventing the engine rotational speed from dropping below the target idling speed so that a transition from the former to the latter can be made in smooth fashion.
According to this previously proposed method, a sharp decline in engine rotational speed from high rpm is dealt with by temporarily halting the downward trend when the rotational speed falls to rpm higher, by a predetermined value, than an upper limit value of the idling speed region. In this way the rotational speed of the engine is made to gradually approach the target idling speed. More specifically, when the engine rotational speed falls below a predetermined speed value higher than the upper limit value of the idling speed region, a current command (control variable) Isa is generated. The value of Isa is decided by the prevailing rotational speed (Ne) of the engine and the difference (.DELTA.Ne, hereafter referred to as a "speed differential") between the present value of rotational speed and the immediately preceding value thereof. The control variable Isa is outputted as the solenoid current command value Icmd for a predetermined period of time (e.g. a fixed time period) Tsa.
According to the previously proposed method described above in which the downward trend in the rotational speed of the engine is temporarily halted by outputting the control variable Isa as the solenoid current command value Icmd for the predetermined time period Tsa, the value of Tsa is preset in dependence upon the engine rotational speed Ne and the speed differential .DELTA.Ne. In other words, with the conventional method of regulating the control valve to give a wider opening in such a manner that the engine rotational speed can make a smooth transition to the target rotational speed at engine idling, control is based upon making a prediction of rotational speed. However, due to slight differences from one internal combustion engine to another, and depending upon the particular vehicle, engine rotational speed may be caused to rise somewhat by the control variable or the downward trend in the rotational speed of the engine may not be fully prevented in an appropriate manner merely by outputting the control variable Isa for the predetermined time period Tsa. With the conventional method, therefore, engine rotational speed cannot always be stabilized at the target idling speed in a smooth manner.